Electrolytic cladding process



Unite ELECTROLYTIC CLADDINGPROCESS N Drawing. Application September 30, 1953 'Serial'No. 383,-401

;5- Claims. :(tll. 204-:39)

Thisinvention relates -to:.the;cladding of :base metals with :corrosion-resistant metals :and, .more particularly, to such- Cladding ;by :electrolysis ,in :a fused; salt bath.

It is well known that such corrosion-resistant metals as titanium and 1;-zirconium ;can be electrolytically deposited on a'base metal cathode immersednimfusedgsalt bath. It is characteristiclof such deposits, however, that they may be readily stripped or jarred loose from the base metal, and in iactthis.characteristicsis.reliedupon toi facilitate lrecovery of :the electrolytic depositinthe production of;massiv e.shapes of'the deposited metal.

ln-our.copending United:States patentapplication Serial No. 358,194, :filed May .28, :1953, :there is described and claimed a :highly .eifectivezmethod ;of forming dense deposits of titanium-zirconium, fhafniurn, :tantalum, vanadium and niobium ,onta: base imetalzcathode The: method of-vsaidzapplicationischaracterized by'a plating-out action whereby :the anode component :o'f the "metal :be deposited is caused 10311385:iHtOSaIldlthldllghjaifusfid I halide salt :bath and zonto a base :metal -;cathode without significant electrolytic decomposition ;of any component of theebath.

We have now 'found {that if the telectrodeposition described and claimed in our aforesaid application a is scarried out using a combinationiofza gspecificmangegofwrelatively high :bath temperatures and .a specific range of cathode-current densities, alloying takes place betweenathe deposited metal and tthe :base cathode metal with the result thatzthere-isiformed ,on the --surface:,of the'base .metal a firmly adherent layer of the :depositeid metal joined to the base metalby a metal-to-metalbond. The fact that this deposited metallayeivis so adherent; asto be virtually rimpossible -to separate .from the base =metal .by ,mere physical distortion- :characterizes :the resulting ,productwitha the mechanical properties :ofsthe'zbase metal and .the ycorrosionrresitant ,properties :of the deposited .metal. a

It will ;he. seen-therefore, zthat jwe have inowudevised-za method ,of cladding ,a hase ,metal with :a firmly adherent deposit. ofqcertainscorrosion-resistant metals,:,to-;-wit: titantum, zirconium, hafnium, tantalum, yanadium and niolbiurn. This method ,of,o.ur,inventionscomprises using :astheanode a solidmetalliferous-form of-such atr-ansition .metal, .using .as the cathode in zthe aforementioned ,platingmut ,procedure a ,base .metal zcapable -;0f forming an "alloy -.with the vcorrosion-resistant .imetal, maintaining the cell voltagehelow tha at which any bath component decomposes with :theevolution of significant amount! of 1a ifreehalogen butsufiieient to establish av cathodeicurrent density of ,2 5 .to ,2. 0.0;-ampere s per .sguare-.decimeter, and maintaining theacell ,bath temperature between 900 :to -1- Q0 'C- w The anode (material which supplies the corrosion-re is t m tdepo ited $911 the basetmeta t thode may somuri e-ar y l d m talliie u if rm sfathe corrosionss s n met l t chga mma bidm i ct aycarhid ao -tlres rr etal or the .metalitself in pure :nr impure ,form. The corrosion-resistant metals useful in practicing our tates latent ice 5 itself as described 'in-the United-States patent application Number 389,191, filed December '14, U53, and now Pa-tent"2;813,069. 'All'of these solid-anode =materialsi-are capable ofsupplying 'the' corrosion-resistant"metal "component which is carried into and-throughthe'bathitodhe base metal cathode.

The base metals which may be used as cathodes in the 1 practice of our invention are all metals having -;a melting point at-least as high as 1060" C. andinclude,

but-are not limited-to, iron, steel, stainlesssteelsymdlybdenum-iron alloys, molybdenumrnickel "alloys, nickelchromium alloys, nickel-copper alloys, "nickel; chromium and-acopper. All of the aforementioned "base metals'have melting points substantially.- above the operating bathtemperatures whichpromote the firm adherence-of the coating to the base-metal by a metal-to-metal'bond. :The

base metal cathode may'have any suitable shape compatible with the geometryof-the-electrolytic cell in which it is cl-ad with the corrosion-resistantmetal.

The electrolytic cell' baths useful in practicing ourQinvention comprise 'fuse'd "halide salts composed' primarily of one or more of the alkali metal halides or alkaline earth metalhalides, ormixtures thereof, iii-admixture with a significant 'but generally minor proportion "of a ihalide 0f the corrosion-resistant metal -to be :deposited from 'the bath. alkaline earth -metal halide, or mixtures thereof, com- I prising the major bath constituentis governed largely 'by .consi'deratiomof the 'melting point and-volatility of the bath. Thus, the bath composition "should 'be such i that The choice 'of alkali ,metal halide or it is not only molten and fluid at temperatures "upwards .ofabout =850'C. butis also not excessively vo'latilized at temperatures upito about l000'C. Although simple halides 'o'f the 'corrosion-resistant metalslare suitableqas the-minor constituent of the bath,'the alkali;metal double fiuoridesof these metals are particularly satisfactory and are-readily available in substantially pure 'form. For example, in the electrodeposition of titanium, which is representative ofthepractice of our invention, the'fuse'd salt bath may be composed, in addition .to the titanium halide, of one or a-mixture of the chlorides, bromides,

iodides and fluorides of alkali metals such .assodium and potassium. he titanium-halide may bea chloride, bromide, iodide or fluoride and maybe a simple halide or a complex halide such as a double fluoride of titanium andan alkalimetal (alsoknown-as analkali 'metalifiuotitanate). The corrosion-resistant metal halide should be present in the bath in the amount of at1least about 5% by weight and generally up'to abouti25% by Weight. ThPI'6S3HCQ'Of fluorine in thebath in theform of (such a double fluoride of an alkali metal andjthe corrosionresist-ant metal, or in the form of a simple'alkali metal fluoride, promotes the formation of larger,Particles of cathodically deposited corrosion-resistant metal th'an'that which is obtained by any one or amixture of the other halides. Except for this special eliect of an added fluoride, the specific composition of the bathlappearswto have no .efiectupon ltheoquality of the electrodeposited metal. Illustrativebath compositions whichareuseful-in the practice of our invention for I the electrodeposit-ion .of titanium, which is representative oftheothercorrosionresistant metals, are set forth in the following table, the

. 3, v V numerical values under each salt heading representing the parts by weight or percentage of each component in the bath:

KzTiFs NaGl K01 NaBr KB! NaI KI 5 95 90 10 45 45 MN 5 30 20 20 10 10 3O l 10 The purity of the bath composition should be as high as possible. The alkali metal halides and alkaline earth metal halides. are readily available in the substantially chemically pure state. The corrosion-resistant metal fluorides can also be obtained or prepared in substantially pure form, but the alkali metal double fluorides of these metals are particularly amenable to purification by simple recrystallization from an aqueous medium. All of these :bath constituents should be used in their substantially anhydrous form, and the anhydrous quality of the bath can be further enhanced by conventional pre-electrolysis a free halogen. Under such cell voltage conditions, the corrosion-resistant metal component of the solid metalliferous anode goes into solution in the bath and is transported through the bath with ultimate deposition on the solid cathode metal. This electrolytic action is of the plating-out type as distinguished from the electrolytic de composition type. Cell voltages useful in practicing our plating-out operation may be as low as 0.7 volt but should not exceed about 3.0 volts. However, in order to effect the cladding of the cathode metal with a metal-to-metal bond between it and the corrosion-resistant metal, the cell voltage should be so adjusted within this range as to establish a cathode current density Within the range of 25 to 200, and preferably to 100, amperes per square decimeter.

Within the aforementioned range of cathode current density, we have found that the deposited corrosion-resistant metal will alloy with the base metal of the cathode when the cell bath temperature is maintained within the range of 900 to 1000 C. When the cathode material is copper, the optimum bath temperature for effective cladding is about 900 C., but for all of the other base metals and alloys mentioned hereinbefore as suitable cathode materials the optimum bath temperature is any temperature within the range of 950 to 1000 C. The alloying of the deposited corrosion-resistant metal with the cathode base metal appears to take place as a result of-ditfusion of one metal into the other or as a result of the formation of a eutectic, or possibly by a combination of these mechanisms. Although We are not certain which of these mechanisms may be the dominant factor in the alloyage of the deposited metal with the cathode metal, it is certain-that it does not result from fusion of the deposited metal or of the cathode metal because the effective bath temperatures of 900 to 1000 C. are well below 'the melting points of all of these metals. Regardless of the theory which may prevail, we have found it to be a fact that such alloyage does take place. The alloy composition formed immediately adjacent the cathode surface is relatively low in its content of the corrosion-resistant metal, but successive deposit layers are progressively higher in their corrosion-resistant metal content so that by thetime a deposited continuous layer about one-sixteenth inch thick has been laid down its surface will be'composed of the substantially pure corrosion-resistant metal.

The electrolysis is carried out under purified argon in which all oxygen, hydrogen, water vapor, nitrogen, and the like have been eliminated by conventional cleaning techniques well known in the art. When anodes composed of the carbides of one of the corrosion-resistant metals are properly made and are used in the form of the densely sintered rods or other shapes as described here inbefore, a condition evidenced by a silvery white appearance at a fracture, the electrolytic reaction proceeds quietly as long as a significant amount of metal carbide remains in the anode structure. In view of the frangible nature of the residual carbon structure, it can be removed readily from the upper unused portions of a long anode by rapping it sharply. Then the unused portion of the same anode is inserted in the bath and the electrolysis is continued. Most efficient utilization of the metal carbide dictates a physical shape such as to provide a relatively large surface area-to-volume ratio. With these conditions properly observed,'at least 90% of the corrosionresistant metal content of the metal carbide will be de posited on the cathode. I

The following specific examples are illustrative of the practice offour invention:

' Example I ditions to a temperature of about 20002l00 C. for a period of one hour. Within a few minutes after reaching this temperature, the vacuum conditions within the heating furnace had reached a steady state and these vacuum conditions were maintained during the remainder of the one-hour heat treatment.

The electrolytic cell consisted of a graphite container in the center portion of which a baifle was positioned, the baflle being pierced with a multiplicity of fine holes over :an area which extended to the bottom of the crucible.

approximately 850 C. Thereupon the heating of the cell was continued under the controlled atmosphere until the salt fused and reached atemperature of 1050 C. 'At this stage a graphite rod was inserted into the bath,

and pre-electrolysis of the bath was begun by applying a voltage of 1.5 voltsbetween the rod as the cathode and the container as the anode; The cell current was initially approximately 15 amperes, but within one to two hours it fell rather abruptly to about /3 of this value,

thus indicating that most of the oxygen-containing impurities had been discharged from the bath. 7 The voltage was then raised to and held at 3.2 volts for about 45 minutes, following which this pro-electrolysis wasstopped and the graphite cathode rod was withdrawn.

The bath was then in anhydrous molten'condition for electrolytic cladding pursuant to the practice of our invention. Accordingly, thetempe ratui'e of the cell was dropped to 950 C., and pellets of the aforementioned titanium carbide were introduced into the annular space betweenthe baffle and the container wall until the space was' filled up to, thebath level. Thesteelpartto be plated, supported by a clamp and shank made of nickel, was inserted -into-ithe bath as the cathode. A voltage of 1.8 volts was applied between this-cathode andthecarbide pellets which functioned as the anode. The cell-current was about 50 amperes and correspondedto a cathode current density of approximately 100 amperes per square decimeter. Electrolysis was conducted for a total of about 30-ampere-honrs after which the clad cathode was withdrawn tocoolin a closed chamber above the cell throughwhich-purified argon was. also keptflowing. The cathode removal chamber was closed with agate valve so that when the cathode had been removed f'rom'the top of the cell-.a second cathode article to be plated could be immediately inserted into the cell, and the cladding of this second article was thus begun witha minimum loss of time. By this procedure-and by us ng the aforementioned bath -itwas possible to clad in this-manner 3,0 such articles before the carbide anode pellets-were so depleted of metalthat the operation had to be discontinued. At this stage the titanium carbide rods were removed'from the cell, the carbon regulus-on the surface of the rods was removed by tumbling, and theremaining solidtitanium carbide cores were used as the carbide pellets charged to the samebath.

Each of the clad cathodes was then soaked-in water for about ,10 minutes, and, thereafter the outer layer of salt and small amounts of loose granular metal were easily stripped off to disclose a shiny smooth continuous layer of substantially pure metallic titanium of about & inch thickness adhering to the underlying steel body by a metal-to-metal bond.

Example II The operation described in Example I was repeated with the exception that the anode consisted of several of the aforementioned titanium carbide rods mounted at equally spaced positions around the cathode. These rods, like the cathode, were inserted in the molten bath after the pro-electrolysis was completed. The results were the same as in Example I.

Example III The operation described in Example I was again repeated except that the anode material consisted of metallic titanium in the form of an anode-shaped ingot obtained by melting under an argon atmosphere of finely divided metallic titainum obtained by a previous but dissimilar electrolytic operation. This anode contained 98% titanium. The results were the same as in Example 1.

Example IV The operation described in Example I was again repeated with the only change being that the cathode shape to be clad was copper, and that the electrolytic cladding bath temperature was maintained at about 900 C.

Example V The operation described in Example I was repeated to effect the cladding of a base metal cathode with each of the metals zirconium, hafnium, tantalum, vanadium and niobium. For each of these cladding metals the carbide and the double fluoride were those of the corresponding cladding metal, and in each operation the base metal was clad with a tightly adhering and continuous layer of the cladding metal.

We claim:

1. A method of forming an electrodeposited coating of a coating metal of the group consisting of titanium, zirconium, hafnium, vanadium, tantalum and niobium on a metal base in which the coating is bonded to the metal base by an alloy consisting of the base metal and the coating metal which comprises: providing an anhydrous fused salt electrolyte consisting essentially of at least 5% by weight of a halide of the coating metal and at least amma V 6 :one halide of *the group consisting of'the alkali metal lzralitles --and the "alkaline "earth metal halides; immersing "in saidmolten electrolyte a-metalliferous anode of said coating metal ;and a cathode of a base metal diflerent from the coating metal and selected from the group consisting of metals having a melting point of at least l000 1C .,;'passing;an electrolyzing current between said anode and saidcathodesufiicient to establish a cathode current density 0'15"25 to 200-amperes per square decimeter and to maintain a cell voltage below that at whichany bath component decomposes with the evolution of a significant amount of;a freefhalogen and cladding the cathode with analley bondedadherent coating'of said coating metalby maintaining the electrolyte temperature above 900 "C. and at, least-sufiicient to 'form an alloy between the coating'metal and the basemetal andbelow the temperature 'at'which'a'liquid alloyforms between the basegmetal and the coating metal andnot above '1000 C. during :said electrolysis.

2. A'method o'f forming an electrodeposited coating of ,a-coating metal of the group consisting of titanium,'zirconiurn,-;hafnium,-vanadiurn,tantalum and niobium on a metal-"base in which-thecoating is bonded to the metal baseby an all-oyjconsisting of the base metal and the coating metal which comprises: providing an anhydrous fused saltelectrolyte consisting essentially of at least 5% by weight of aha'lide of the coating metal and at least one halide :of-the group consisting of-the alkali metal halides and the alkaline earth metal halides; immersing lnSElld molten electrolyte (l) a metalliferous anode of said coating metal and (2) a cathode of a base metal different from the coating metal and selected from the group consisting of iron, steel, molybdenum-iron alloys, molybdenumnickel alloys, nickel and chromium; passing an electrolyzing current between said anode and said cathode sufficient to establish a cathode current density of 25 to 200 amperes per square decimeter and to maintain a cell voltage below that at which any bath component decomposes with the evolution of a significant amount of a free halogen and cladding the cathode with an alloy bonded adherent coating of said coating metal by maintaining the electrolyte temperature above 950 C. and at least suificient to form an alloy between the coating metal and the base metal and below the temperature at which a liquid alloy forms between the base metal and the coating metal and not above 1000 C. during said electrolysis.

3. A method of forming an electrodeposited coating of a coating metal of the group consisting of titanium, zirconium, hafnium, vanadium, tatalum and niobium on a metal base in which the coating is bonded to the metal base by an alloy consisting of the base metal and the coating metal which comprises: providing an anhydrous fused salt electrolyte consisting essentially of at least 5% by weight of a halide of the coating metal and at least one halide of the group consisting of the alkali metal halides and the alkaline earth metal halides; immersing a metalliferous anode of said coating metal and a steel cathode in said molten electrolyte; passing an electrolyzing current between said anode and said cathode sufiicient to establish a cathode current density of 25 to 200 amperes per square decimeter and to maintain a cell voltage below that at which any bath component decomposes with the evolution of a significant amount of a free halogen and cladding the steel cathode with an alloy bonded adherent coating of said coating metal by maintaining the electrolyte temperature above 950 C. and at least sufiicient to form'an alloy between the coating metal and the cathode metal and below the temperature at which a liquid alloy forms between the cathode metal and the coating metal and not above 1000" C. during said electrolysis.

4. A method of forming an electrodeposited titanium coating on a metal base in which the coating is bonded to the metal base by an alloy consisting of the base metal and the coating metal which comprises: providing an anhydrous fused salt electrolyte consisting essentially of atleast 5% by weight of a halide of the coating metal andat least one halide of the group consisting of the alkali metal halides and the alkaline earth metal halides; immersing a metalliferous anode of titanium and a steel cathode in said molten electrolyte; passing an electrolyzing current between said anode and said cathode sufiicient to establish a cathode current density of 25 'to 200 amperes per square decimeter and to maintain a cell voltage below that at which any bath component decomposes with the evolution of a significant amount of free halogen and cladding the cathode with an alloy bonded adherent coating of said coating metal by maintaining the electrolyte temperature above 950 and at least suflicient to form an alloy between the titanium and the cathode metal and below the temperature at which a liquid alloy forms between the cathode and the titanium and not above 1000 C. during said electrolysis.

5. A method of forming an electrodeposited coating of a coating metal of the group consisting of titanium, zirconium, hafnium, vanadium, tantalum and niobium on a metal base in which the coating is bonded to the metal base by an alloy consisting of the base metal and the coating metal which comprises: providing an anhydrous fused salt electrolyte consisting essentially of at least 5% by weight of a halide of the coating metal and at least one halide of the group consisting of the alkali metal halides and the alkaline earth metal halides; immersing a metalliferous anode of said coating metal and a cathode of copper in said molten electrolyte; passing an electrolyzing current between said anode and said cathode sulficient to establish a cathode current density of 25 to 200 amperes per square decimeter and to maintain a cell voltage below that at which any bath component decomposes with the evolution of a significant amount of a free halogen and cladding the cathode with an alloy bonded adherent coating of said coating metal by maintaining the electrolyte temperature above 900 C. and at least sufiicient to form an alloy between the coating metal and the cathode metal and below the temperature at which a liquid alloy forms between the cathode metal and the coating metal and not above 1000 C. during said electrolysis.

References Cited in the file of this patent UNlTED STATES PATENTS 1,535,339 Peacock Apr. 28, 1925 1,845,978 Hosenfeld Feb. 16, 1932 1,927,773 Chittum Sept. 19, 1933 1,933,319 Driggs et al Oct. 31, 1933 2,715,093 Senderofi et a1. Aug. 9, 1955 OTHER REFERENCES Kroll et al.: U. S. Bureau of Mines, Report of Investigation RI 4915, November 1952, pp. 17-26.

Mann et al.: Transactions Electrochemical Society, vol. 45 (1924), pp. 493-508.

Journal of the Electrochemical Society, August 1952, vol. 99, No. 8, pages 223C and 224C.

U S. DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No\: 2,828,251 March 25, 1958 Merle Ee Sibert et ale It is hereby certified that error appears inthe printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected belowo Column l line 50, for "eorrosion=-resitant" read oorrosion= reeietant line 62, before "significant" insert a g column 2 line 11, for serial number "389,191" read we 3.98 191. column 4, line 5, for "carbides" read carbide =9; line 29, after "105" strike out the comma} column 5, line 48, for titainum" read titanium column 6 line 1.3, for "alley" read alloy line 499 for "tatalum" read tantalum Signed and sealed this 20th day of May 1958a (SEAL) Attest:

KARL AXLINE ROBERT c. WATS 0N Atteelting Officer Conmissioner of Patents 

1. A METHOD OF FORMING AN ELECTRODEPOSITED COATING OF A COATING METAL OF THE GROUP CONSISTING OF TITANIUM, ZIRCONITUM, HAFNIUM, VANADIUM, TANTALUM AND NIOBIUM ON A METAL BASE IN WHICH THE COATING IS BONDED TO THE METAL BASE BY AN ALLOY CONSISTING OF THE BASE METAL AND THE COATING METAL WHICH COMPRISES: PROVIDING AN ANHYDROUS FUSED SALT ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST 5% BY WEIGHT OF A HALIDE OF THE COATING METAL AND AT LEAST ONE HALIDE OF THE GROUP CONSISTING OF THE ALKALI METAL HALIDES AND THE ALKALINE EARTH METAL HALIDES, IMMERSING IN SAID MOLTEN ELECTROLYTE A METALLIFEROUS ANODE OF SAID COATING METAL AND A CATHODE OF A BASE METAL DIFFERENT FROM THE COATING METAL AND SELECTED FROM THE GROUP CONSISTING OF METALS HAVING A MELTING POINT OF AT LEAST 1000* C., PASSING AN ALECTROLYZING CURRENT BETWEEN SAID ANODE AND SAID CATHODE SUFFICIENT TO ESTABLISH A CATHODE CURRENT DENSITY OF 25 TO 200 AMPERES PER SQUARE DECIMETER AND TO MAINTAIN A CELL VOLTAGE BELOW THAT A WHICH ANY BATH COMPONENT DECOMPOSES WITH THE EVOLUTION OF A SIGNIFICANT AMOUNT OF A FREE HALOGEN AND CLADDING THE CATHODE WITH AN ALLEY BONDED ADHERENT COATING OF SAID COATING METAL BY MAINTAINING THE ELECTROLYTE TEMPERATURE ABOVE 900*. AND AT LEAST SUFFICIENT TO FORM AN ALLOY BETWEEN THE COATING METAL AND THE BASE METAL AND BELOW THE TEMPERATURE AT WHICH A LIQUID ALLOY FORMS BETWEEN THE BASE METAL AND THE COATING METAL AND NOT ABOVE 1000*C. DURING SAID ELECTROLYSIS. 